Surface acoustic wave filters to be employed in front end circuits of wireless telecommunications systems must have a particularly low insertion loss in their transmission band (passband).
To achieve the required relatively large bandwidth, the surface acoustic wave filters are formed on piezoelectric substrates such as LiTaO3 with leaky wave properties. In such substrates, leaky wave losses occur, which can be reduced under certain conditions relevant to the propagation of the surface wave.
It is known from the publication DE 19641662 that a low insertion loss in the passband of the filter is achievable at a defined angle of intersection φ of the piezoelectric substrate and at a defined layer thickness h of the electrodes. The parameters φ and h at which the optimal insertion loss occurs are dependent on the material of the metallization.
Achieving a low insertion loss by the adjustment of a defined metallization ratio η of the substrate surface is known from the publication US 2003/0117240.
Forming the electrodes from several layers, for instance, a sequence of Al and Cu layers, is known. For reasons of process technology, however, a uniform layer structure with a uniform layer thickness and a uniform metallization ratio is generally selected for the metallization of the electrodes.
Forming some electrode fingers in an electroacoustic transducer with a higher layer thickness than the other electrode fingers is known from the publication U.S. Pat. No. 5,073,763. Thereby a directed acoustic reflection or the unidirectional emission of the wave can be achieved.
It may be possible to maintain the required insertion loss at certain critical points within the filter's passband, especially at the passband edges of a filter, only with great difficulty.